feat: Expand expression for join plan

[Gusabary]

solves #24157

while i am not sure whether it's really desired behavior to expand expression for join, for example test_join_on_nth_error would fail with this change

can add more tests if this behavior change is aligned eventually

[orlp]

def test_join_on_nth_error() -> None:
    df = pl.DataFrame({"x": [1]})
    df2 = pl.DataFrame({"x": [1], "y": [2]})
    with pytest.raises(
        InvalidOperationError,
    ):
        df.join(df2, on=pl.first())

What behavior does this now have in your PR?

[Gusabary]

What behavior does this now have in your PR?

df = pl.DataFrame({"x": [1]})
df2 = pl.DataFrame({"x": [1], "y": [2]})
print(df.join(df2, on=pl.first()))
┌─────┬─────┐
│ x   ┆ y   │
│ --- ┆ --- │
│ i64 ┆ i64 │
╞═════╪═════╡
│ 1   ┆ 2   │
└─────┴─────┘

df = pl.DataFrame({"a": [1]})
df2 = pl.DataFrame({"x": [1], "y": [2]})
print(df.join(df2, on=pl.first()))
┌─────┬─────┐
│ a   ┆ y   │
│ --- ┆ --- │
│ i64 ┆ i64 │
╞═════╪═════╡
│ 1   ┆ 2   │
└─────┴─────┘